Tom Glasgow: Trees and roots need room to grow

This large growing oak was given no chance to develop a normal spreading root system. Both stability and overall tree health are compromised as a result.

Contributed photo

Published: Friday, May 2, 2014 at 07:44 PM.

A number of factors may be considered in determining whether a large, questionable street or parking lot tree should be removed or retained. One of the most important discussion points should be whether or not the tree in question has been provided with adequate space for trunk and root development.

Mature trees are not held in place by deep taproots. Instead, the vast majority of tree species are stabilized by relatively shallow and wide spreading root systems. Most root growth will occur in the top two feet or so of the soil profile.

When adequate space has not been provided, trees tend to respond with poor color, canopy dieback and shortened lifespans. In addition, large trees with inadequate rooting space will be more likely to fall over in storms. Many of the mature street trees knocked over by Bertha and Fran in 1996, for example, were growing in narrow street/sidewalk planting strips.

Current recommendations from forestry and horticulture experts tend to go far beyond the space typically provided for our street and parking lot trees. As noted in the Virginia Urban Street Tree Selector (http://dendro.cnre.vt.edu/treeSelector), “Large shade trees in downtown settings rarely, if ever, have sufficient soil volume to grow to their full potential size.”

Ed Gilman of the University of Florida recommends 1,000 cubic feet – assuming a soil depth of three feet – for a tree with a mature trunk diameter of 16 inches. For a trunk diameter of 24 inches, the recommendation increases to 1,700 cubic feet. (See http://hort.ifas.ufl.edu/woody/street-trees.shtml and click on Urban Design.) If the available soil depth is less than three feet, the width of the space must be increased.

Other models are available in addition to Gilman’s specifications. For example, the Lindsey & Bassuk model indicates that a Zelkova serrata with a 24-inch diameter at breast height and a crown spread of 50 feet, growing in soils typical of Blacksburg, Virginia would need about 2,200 cubic feet of soil. A model developed by James Urban et al. calls for about 1,500 cubic feet. Still another model, the Natural Forest method that is based on upland hardwoods in the eastern U.S., would call for between 1,700 and 4,100 cubic feet.

Simple calculations show that, regardless of the method used, most communities fall short in providing rooting space for large trees. For example, using Gilman’s formula of 1,700 cubic feet for a trunk diameter of 24 inches (breast height), and assuming a soil depth of 3 feet, you would need a sidewalk or parking lot cutout close to 23 feet by 25 feet (3 x 23 x 25 = 1,725) for best results.

A number of factors may be considered in determining whether a large, questionable street or parking lot tree should be removed or retained. One of the most important discussion points should be whether or not the tree in question has been provided with adequate space for trunk and root development.

Mature trees are not held in place by deep taproots. Instead, the vast majority of tree species are stabilized by relatively shallow and wide spreading root systems. Most root growth will occur in the top two feet or so of the soil profile.

When adequate space has not been provided, trees tend to respond with poor color, canopy dieback and shortened lifespans. In addition, large trees with inadequate rooting space will be more likely to fall over in storms. Many of the mature street trees knocked over by Bertha and Fran in 1996, for example, were growing in narrow street/sidewalk planting strips.

Current recommendations from forestry and horticulture experts tend to go far beyond the space typically provided for our street and parking lot trees. As noted in the Virginia Urban Street Tree Selector (http://dendro.cnre.vt.edu/treeSelector), “Large shade trees in downtown settings rarely, if ever, have sufficient soil volume to grow to their full potential size.”

Ed Gilman of the University of Florida recommends 1,000 cubic feet – assuming a soil depth of three feet – for a tree with a mature trunk diameter of 16 inches. For a trunk diameter of 24 inches, the recommendation increases to 1,700 cubic feet. (See http://hort.ifas.ufl.edu/woody/street-trees.shtml and click on Urban Design.) If the available soil depth is less than three feet, the width of the space must be increased.

Other models are available in addition to Gilman’s specifications. For example, the Lindsey & Bassuk model indicates that a Zelkova serrata with a 24-inch diameter at breast height and a crown spread of 50 feet, growing in soils typical of Blacksburg, Virginia would need about 2,200 cubic feet of soil. A model developed by James Urban et al. calls for about 1,500 cubic feet. Still another model, the Natural Forest method that is based on upland hardwoods in the eastern U.S., would call for between 1,700 and 4,100 cubic feet.

Simple calculations show that, regardless of the method used, most communities fall short in providing rooting space for large trees. For example, using Gilman’s formula of 1,700 cubic feet for a trunk diameter of 24 inches (breast height), and assuming a soil depth of 3 feet, you would need a sidewalk or parking lot cutout close to 23 feet by 25 feet (3 x 23 x 25 = 1,725) for best results.

When the decision is made to remove a declining, over mature tree that among other problems has not been provided adequate rooting and trunk space, efforts should be made to replant with something more appropriate to the site.

Smaller species can be selected for the limited space planting sites, with larger growing species reserved for residential lawns, parks or other sites that provide plenty of room. The result can be a much safer and healthier community forest for future generations.

Tom Glasgow is the Craven County Extension director. Contact him at tom_glasgow@ncsu.edu.